Dear Dr. SETI:Do you know how much the Iridium satellites will influence the amateur
radio astronomy and SETI?
I've learned that some radio observatories (in the north hemisphere) had
signed agreements with Motorola.
But, what will happen with those who work in the south, especially with the
amateurs?

Rolando, Peru

The Doctor Responds:
I hate to cast a pall upon your amateur SETI activities, Rolando, but the situation is pretty bleak. Over the past year Motorola launched an array of 66 Personal Communications System (PCS) satellites into low-earth orbit, to provide global telephone and internet access. These satellites, which become operational in September 1998, have downlinks in the 1621.25 to 1626.5
MHz band. Many radio astronomers observe the
hydroxyl radical radiation line at 1610.6-1613.8 MHz, a segment internationally allocated to radio astronomy on a
primary basis. These frequencies are used by astronomers to study the
distribution of the hydroxyl radical, one of the most common
interstellar molecules, enabling them to investigate a wide range of
issues including the evaporation of comets and the birth and death of
stars.

Even though Iridium does not actually violate the International Telecommunications Union allocations, It is entirely possible
that radio astronomy activities around that frequency will be impacted by adjacent-channel interference. The agreement to which you refer was reached between the European Science Foundation (ESF) and Iridium LLC, operators of the Motorola satellites. A press release issued by ESF on 13 August 1998 states:

The agreement signed by the ESF, on behalf of its associated Committee
on Radio Astronomy Frequencies (CRAF), and Iridium LLC is the result
of six months of intense negotiations. Under its terms, Iridium
guarantees Europe's radio astronomers 24 hours a day of 'unpolluted'
observation time from 1 January 2006. Both
parties are also committed to reaching a further agreement by 1 March
1999 on transitional arrangements, covering the number of hours each
day, during which Iridium unwanted emissions are to be restricted and
an agreed maximum interference level at other times, for the period 1
March 1999 to 31 December 2005. For the six months from Iridium's
start-up in September 1998 until 1 March 1999, the satellite company
has agreed to keep emission levels below harmful interference levels
as requested by the radio astronomers. However, in practice, even
these levels imply a concession by radio astronomers to
satellite-enabled services as the sensitivity of current state-of-the
art radio astronomy equipment would imply that they should be set
considerably lower.

In addition, under the terms of the agreement, both parties will
continue to work together to find adequate and technically practical
solutions for reducing both the out-of-band emissions of the Iridium
satellite system and the susceptibility of radio astronomy equipment
to these emissions.

After signing the agreement, ESF Secretary General, Professor Enric
Banda commented: "This is an important agreement for radio astronomy
and provides welcome guarantees. Radio astronomy, as a passive
service, is uniquely vulnerable to radio interference and CRAF's
success, in representing the interests of Europe's different radio
astronomy observatories during these negotiations, has once again
demonstrated the value for Europe's scientific community of
cooperation and of speaking with one voice."

He added: "The agreement also underlines the willingness of CRAF and
Europe's radioastronomers to work constructively with the growing
number of satellite-enabled companies to find sustainable technical
solutions that will allow science and industry to continue to
profitably coexist in space."

However, despite this agreement, interference from satellites remains an
increasing threat to astronomy. "This is not an isolated problem,"
said Dr Jim Cohen, CRAF's Chairman. "The number of cases of
interference to radio astronomy from satellites is growing steadily.
Unless the protection of radio astronomy is taken into account early
in the design of new satellite systems our science could face a
difficult future."

Although this agreement appears to provide some relief to European radio astronomers, I see no such protection being proposed for the rest of the world. Satellite interference is not new to the SETI community. Radiation at 1575 MHz from the constellation of 24 Global Positioning System (GPS) satellites is a well known pollutant in the water-hole spectrum. Fortunately, observations around the 1420 MHz hydrogen line, and the hydroxyl component
around 1660 MHz, should not be affected by Iridium. As those are the two most popular
frequencies for SETI, the impact on our own Project Argus search should be tolerable. However, Iridium is
only the beginning, and other, competing PCS
satellite constellations are planned. Things can only get worse.

ADDENDUM:

As of the third quarter of 2000, the Iridium commercial venture has gone bankrupt, and the satellites are scheduled to be de-orbited (i.e., burned up in the Earth's atmosphere) over a two-year period. I suppose this is good news for radio astronomy and SETI, and bad news for Iridium's investors. Yet, despite Iridium posting a $5 Billion loss, other constellations of low Earth orbit personal communications satellites are planned, and many of them propose to operate in or near the prime radio astronomy bands. So Iridium is simply Round One of an ongoing battle between science and commerce.